In congestive heart failure due to mitral regurgitation (MR) and idiopathic dilated cardiomyopathy (IDCM) the dilated ventricle undergoes dramatic decreases in the velocity of myocardial shortening and power output. These changes can be seen at the cross-bridge level, as increased cross-bridge force-time integral and decreased myofibrillar ATPase activity. Since both normal and failing human hearts contain predominantly V3 myosin isoform, and since myosin ATPase activity from these preparations is unchanged, it is unlikely that myosin isoform shifts cause these kinetic changes. In light of the observed changes in cross-bridge performance, our objectives are: 1) to determine the molecular basis for the cause of this increase in cross-bridge force-time integral in failing hearts; and 2) to determine the mechanical parameters at the molecular level that are altered to produce the observed increase in the cross-bridge force-time integral. The prime target for the cause of the alteration in cross-bridge function found in failing hearts is an isoform shift in the thin filament protein troponin T (TnT) where there is a shift from the adult isoform, TnT1, to the fetal isoform, TnT2. In addition to correlative studies of the relationship between %TnT2 (TnT/TnT1 X 100) and the cross-bridge force- time integral, we will test for causality TnT2 substitution on thin filaments. The cross-bridge force-time integral is the product of the unitary force (Funi) developed during the molecular power stroke and the time during which the strongly bound cross-bridge develops the unitary force (Tatt). We will measure these parameters directly using the laser light trap technology where myosin from non failing (NF) and failing (MR, IDCM) is used with bare actin, tropomyosin decorated actin and with troponin- tropomyosin decorated actin. With regard to troponin-tropomyosin decorated actin, TnT2 rich and TnT2 poor troponin will be used. For the first time Funi and Tatt for failing and non failing human myosin will be measured. The strength of the approach resides in our ability: 1) carry out experiments directly and on simplified human myocardium; 2) to measure the cross-bridge force time integral and ATPase activity on intact and skinned strips from the same non-failing and failing hearts; 3) to substitute thin filament proteins thus allowing us to prove or disprove the causal nature of the %TnT2 effect on the cross-bridge force-time integral and ATPase; 4) to measure unitary force and attachment time of single myosin molecules in contact with bare or appropriately decorated actin filaments using the optical trap system.